Is Seth's voice okay?

I've been watching the show from the start, and I've noticed on later seasons that some characters voiced by Seth (mostly Peter and sometimes Brian. Dr Hartman once that I remember) sound a bit "airy" (I don't know how to describe it in English), like if Seth's voice hurts during recording. Is he okay? Is it just a style thing? Did something happen that I missed? I can't help but notice it

My safest bet is age. Seth is 51 and we can't expect him to sound like he was in his 20s, but I think it changed so instantly you can't help but wonder. He's still a great singer, but Peter's voice scratch my brain in a way I can't quite pinpoint